Method and apparatus for processing wafers

ABSTRACT

An apparatus for processing wafers one at a time. The apparatus has a vacuum chamber  1  into which wafers are loaded through a pair of loadlocks  3, 4  which are spaced one above the other. A robot within the vacuum chamber  1  has a pair of gripper arms  22, 29  which are moveable along and rotatable about a vertical axis  23  so as to be moveable between the loadlocks  3, 4  and a wafer processing position. Each of the loadlocks  3, 4  has a vertically moveable portion  8, 26  which is moveable away from the remainder of the loadlock to provide access in a horizontal plane for one of the gripper arms  22, 29.

This application is a divisional of application Ser. No. 09/293,939,filed on Apr. 19, 1999, now U.S. Pat. No. 6,350,097 the entire contentsof which are hereby incorporated by reference and for which priority isclaimed under 35 U.S.C. §120.

FIELD OF THE INVENTION

A present invention relates to a method and apparatus for processingwafers. The invention has particular application to ion implantationchambers for semiconductor wafers.

BACKGROUND OF THE INVENTION

In such ion implantation chambers, a wafer is scanned across an ion beamto introduce controlled doses of impurities into the wafer. The chamberin which the wafer is processed is evacuated.

In order to load the wafers into the vacuum chamber, a loadlock chamberis used to preserve the vacuum while loading wafers from the outsideatmosphere. The loadlock chamber has an external valve to seal theloadlock chamber from the external atmosphere and an internal valve toseal the loadlock chamber from the vacuum chamber. With the internalvalve closed and the external valve open, the wafer is loaded into theloadlock chamber from the atmospheric side. The external valve is thenclosed and the loadlock chamber is evacuated before the internal valveis opened and the wafer is transported into the vacuum chamber forprocessing. An example of such a loadlock is disclosed in EP-A-604,066.

In order to make most efficient use of the ion beam, and thus increasethe throughput of the apparatus, the loading and unloading of the wafersinto and out of the vacuum chamber must be done as quickly as possible.The present invention aims to improve the performance of the apparatusin this respect.

SUMMARY OF THE INVENTION

According to the present invention, an apparatus for processing waferscomprises a vacuum chamber in which the wafers are serially processed ata wafer processing position, two loadlocks through which the wafers areloaded into the vacuum chamber, and a mechanism for transporting thewafers from the loadlocks to the wafer processing position, eachloadlock having an outer valve which is selectively operable to seal theloadlock from the external atmosphere, an inner valve which isselectively operable to seal the loadlock from the vacuum chamber, and aport for evacuation and pressurisation of the loadlock, wherein oneloadlock is positioned above the other.

The use of two loadlocks which are preferably single wafer loadlocksallows wafers to be transported in parallel through the two loadlocks.Preferably a gripper arm is provided which is rotatable about an axis toaccess the loadlocks, and both loadlocks are positioned at the sameradial distance from this axis. This allows the mechanisms for loadingand unloading both loadlocks on one side of the loadlocks to sharecertain common parts. Preferably, the loadlocks are positioned onesubstantially directly above the other to allow this to be achieved withlittle or no increase in the footprint of the apparatus.

The internal and external valves of each loadlock can be slit valves inwhich a gate member is raised and lowered to uncover a slit allowingaccess to the loadlock chamber. Indeed, such valves are preferred forthe external valves. However, preferably, each loadlock is a two-partstructure, the two parts being movable together in a directionsubstantially perpendicular to the plane of the wafer to seal andprovide the inner valve, and being movable apart in the oppositedirection to break the seal and allow access to the wafer by a gripperarm pivoted about a single axis substantially perpendicular to the planeof the wafer.

To provide a more compact structure, it is preferred that the upperloadlock has an inner valve which is operable by upward movement of atop part, and the lower loadlock has an inner valve which is operable bydownward movement of a lower part.

The wafers from both loadlock chambers can be picked up and set down bya robot which requires only axial motion in the direction of the axisabout which the gripper arm is pivoted, and rotational motion about thisaxis. In fact, in order to allow a processed wafer to be loaded into theloadlock while an unprocessed wafer is being unloaded, a second gripperarm will be provided which is axially movable together with the firstgripper arm. The second gripper arm is either disposed on the oppositeside of the axis to the first gripper arm and is rotatable with thefirst gripper arm, or is positioned immediately below the first gripperarm and is rotatable about the axis independently of the first gripperarm. In the second case, which is preferred as it offers greaterflexibility, the robot in the vacuum chamber is a three axis robot,having one linear and two rotational axes. This is advantageous over aconventional four axis robot as each additional axis required in avacuum chamber increases the cost and the maintenance of the apparatus.

The robot forms an independent aspect of the present invention which canbe broadly defined as a robot for transferring planar members from onelocation to another, the robot comprising a pair of coaxial shaftsextending, in use, perpendicular to the plane of the planar members,each shaft having a gripper for gripping a planar member, the two shaftsbeing rotatable independently about the axis and movable together alongthe axis, and a source of motive power for providing the axial androtational motion.

The coaxial shafts also preferably contain air ducts for the supply ofair to a pneumatic mechanism on each gripper arm for opening and closingthe gripper arm. The coaxial shafts are preferably axially movabletogether by means of a lead screw.

The invention also extends to a method for loading wafers from anatmospheric source to a processing position within a vacuum chamber, themethod comprising the steps of loading a wafer into a first loadlockchamber at atmospheric pressure, closing an external valve on theloadlock chamber and evacuating the loadlock chamber, opening aninternal valve between the loadlock chamber and a vacuum chamber,transporting the wafer to the processing position, and repeating theprocess for a second loadlock chamber directly below the first loadlockchamber. Preferably access to the loadlocks requires a gripper armrotatable about an axis, and both loadlocks are positioned at the sameradial distance from this axis. The second loadlock chamber ispreferably substantially directly below the first.

The transportation of the wafer to the processing position is preferablydone by a robot having a gripper arm which is elevationally moveablealong an axis which is substantially parallel to the direction in whichthe loadlock chambers are separated, and is rotatable about this axis,the method further comprising the steps of moving the gripper arm to afirst elevational location aligned with the first loadlock chamber,rotating the gripper arm about the axis into the first loadlock chamberand picking up a wafer from the first loadlock chamber, rotating thegripper arm about the axis to transport the wafer to the processingposition, depositing the wafer at the processing position, moving thegripper arm axially to a second elevational location aligned with thesecond loadlock chamber, and repeating the gripping, moving anddepositing operations for a wafer from the second loadlock chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a method and apparatus in accordance with the presentinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic cross-section from one side through the twoloadlock chambers and a portion of the vacuum chamber;

FIG. 2 is a schematic plan view of the arrangement shown in FIG. 1;

FIG. 3 is a perspective view of the upper loadlock with the lid valveremoved;

FIG. 4 is a cross-section of the drive mechanism for the two gripperarms in the vacuum chamber; and

FIG. 5 is a throughput graph showing the movements of the variouscomponents of the apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ion implantation apparatus is broadly the same as that disclosed inWO99/13488.

The apparatus comprises a vacuum chamber into which wafers are loadedindependently onto an electrostatic chuck (hereafter referred to as ane-chuck). In operation an individual wafer is electrostatically clampedon the e-chuck and is held vertically and is scanned by a horizontallyscanning ion beam.

The arm which supports the e-chuck extends out of the vacuum chamber andis supported by a linear motion mechanism for reciprocably moving thee-chuck vertically so that the entire surface of a wafer on the e-chuckis scanned by the ion beam. The linear motion mechanism itself ismounted so as to be rotatable about a horizontal tilt axis which allowsthe angle between the wafer and the ion beam to be varied. The e-chuckis further provided with a mechanism for rotating the wafer about anaxis passing through a centre of the wafer and perpendicular to theplane of the wafer. The mechanism thus far described is as that shown inWO99/13488. Further, the arm itself is independently rotatable about thehorizontal tilt axis through 900 so that it can be moved from thevertical scanning position to a horizontal loading position.

The arrangement for loading the wafers into the vacuum chamber of thepresent invention is shown in FIGS. 1 and 2. The apparatus broadlycomprises the vacuum chamber 1 containing the e-chuck (not shown), aloadlock assembly 2 comprising an upper loadlock 3 and a lower loadlock4, and an external atmospheric portion 5. The upper loadlock 3 isdirectly above the lower loadlock 4 in the sense that the wafersretained in the two loadlocks have their centres on the same verticalaxis.

In the external atmospheric portion 5 are a number of magazines whichprovide a source of a wafers to be treated in the ion implantationapparatus, and receive the treated wafers from the ion implantationapparatus.

The loadlock assembly 2 comprises a loadlock housing 6 which has acentral plate 7 separating the upper 3 and lower 4 loadlocks. The upper3 and lower 4 loadlocks are positioned as close together as possible inthe vertical direction to minimise the movement required to load andunload wafers from both loadlocks. The upper loadlock 3 is provided witha lid valve 8 which is elevationally movable by a cam mechanism 9mounted directly above the upper loadlock 3. A bellows 10 provides avacuum seal for the cam actuator 9, and a spring 11 provides a degree ofpreloading for the lid valve 8, and absorbs any dimensional tolerancesbetween the lid valve 8 and the housing 6. To provide access for a wafer12 into the vacuum chamber, the lid valve 8 is raised to the positionshown in FIG. 1 allowing lateral access to the wafer on the dependingfeet for a gripper arm as described below. In FIG. 1 the upper loadlock3 is shown open to the vacuum chamber 1 with the wafer being in theprocess of being removed into the vacuum chamber. Access to theatmospheric side of the loadlock is provided by a slit valve 13 in whichthe gate element 14 can be raised and lowered on an activator 13A inorder to seal across a slit 15 through which the wafer can enter theupper loadlock 3.

The mechanism for loading and unloading a wafer 12 from the upperloadlock 3 is shown in more detail in FIG. 3. It should be understoodthat this figure is schematic, in the sense that it shows both the valveto the vacuum chamber 1 and the valve to the atmosphere 5 open and themechanisms for transferring the wafer from either side being deployedinto the loadlock. Of course, in practice, only one valve will be openat any one time, and only one of the deployment mechanisms will be inplace.

An end effector 16 of an atmospheric robot is shown projecting throughthe slit 15. Within the loadlock, the end effector is represented by apair of parallel fingers 17, but in practice will project beneath thewafer 12 shown in outline only in FIG. 3, so as to support the wafer. Aloadlock carrier 18 is provided to support the wafer in the loadlock.The loadlock carrier 18 has an outer profile which substantially matchesthe circular profile of the wafer. The opposite side of the loadlockcarrier 18 has straight sided recess 19 which is shaped so as to allowthe end effector 16, 17 to pass through the loadlock carrier from aboveas will be described. The loadlock carrier 18 has an upwardly projectingflange which leads up to a bracket 20 with which it is integral. Thisbracket 20 is rigidly fixed to the lid valve 8, so that the wholeloadlock carrier 18 moves up and down with the lid valve 8. Three feet21 are provided on the upper surface of the loadlock carrier so as toreceive the wafer. When the lid valve 8 is raised, the loadlock 3 can beaccessed by a gripper arm 22 moving in a horizontal plane about avertical axis 23.

In order to place a wafer 12 on the loadlock carrier 18, the endeffector 17 carrying a wafer 12 is moved through the slit 15 as shown inFIG. 3. The end effector is then moved downwardly through the recess 19in the loadlock carrier 18 until the wafer is supported by the threefeet 21. The end effector is then moved further downwardly so as to beclear of the wafer 12 and is then withdrawn through the slit 15. All ofthis is done with the lid valve 8 in its lowered or closed position.Once the loadlock chamber has been evacuated, the lid valve 8 is raised,bringing the loadlock carrier 18 and wafer 12 with it. The gripper arm22 is then swung into the position shown in FIG. 3, and is then moveddownwardly, or the loadlock carrier 18 is moved upwardly so that it cangrip the edge of the wafer 12 and withdraw it from the loadlock carrier.

The lower loadlock 4 has a similar design to the upper loadlock, in thatthe cam mechanism 24 and slit valve 25 are of the same construction, butin an inverted configuration. In the lower loadlock, there is no need toprovide a the loadlock carrier to support the wafer, as the wafer can bedirectly supported by feet on the upper surface of lower lid valve 26.The lower lid valve will need a recess of similar shape to the recess 19between the feet to allow the end effect to place the wafer on the feetand be withdrawn.

In FIG. 1 the loadlock is shown in its raised/closed position in whichthe lower lid valve 26 seals around its periphery with the housing 6thereby providing a seal between the lower loadlock 4 and the vacuumchamber 1 and defining a sealed loadlock chamber 27 between the lowersurface of plate 7 of the housing 6 and the upper surface of lower lidvalve 26. The volume of the loadlock chambers of the upper 3 and lower 4loadlocks is kept to a minimum to minimise the pumping and ventingrequired.

In the configuration of FIG. 1 and with slit valve 25 open, a wafer canbe loaded into the loadlock chamber 27 and is supported by the feet onthe lower lid valve. The slit valve 25 is then closed and the loadlockchamber 27 is evacuated through evacuation port 28. The lower lid valve26 can then be lowered breaking the seal on the vacuum chamber side andproviding access to the loadlock chamber 27 from the vacuum chamber 1.

The robot mechanism for transferring the wafers from the loadlockmechanism 2 to the e-chuck will now be described in more detail. Inaddition to the gripper arm 22 shown in FIG. 3, which will subsequentlybe referred to as the lower gripper arm, the robot further comprisesupper gripper arm 29 of the same construction. The two arms are mountedadjacent to one another so as to be movable together along a verticalaxis 23 and rotatable independently about the vertical axis 23.

The mechanism for operating the gripper arms is shown in FIG. 4. Thelower gripper arm 22 is attached via hub 30 to an inner shaft 31. Theupper gripper arm 29 is attached via hub 32 to an outer shaft 33. Theinner shaft 31 is rotated by a motor 35, while the outer shaft 33 isrotated by motor 36. Vacuum seals for the two shafts are provided byferro-fluidic seals. Air ducts 37, 38 allow the transmission of air tothe gripper arms for the pneumatic opening and closing operations ofthese arms. A third motor 39 rotates a feed screw shaft 39A to providethe axial movement of the two gripper arms 22, 29 together along theaxis 23.

The purpose having the pair of gripper arms 22, 29 is that when one isunloading a wafer at a particular location, the other can immediatelyload a wafer at that location without having to wait for the first oneto return with a further wafer for loading. The e-chuck may be at thesame elevational height as one of the loadlocks 3, 4, such thatelevational movement of the gripper arms 22, 29 is only required whenmoving wafers between the e-chuck and the loadlock which iselevationally offset from the e-chuck. On the other hand, the e-chuckmay be elevationally between the two loadlocks, requiring a smallerelevational movement of the gripper arms each time a wafer istransferred.

The entire loading/unloading operation of this apparatus will now bedescribed with particular reference to FIG. 5. The key to this figure isthat five components, namely e-chuck (c), top arm, lower arm, toploadlock (LU), lower loadlock (LL) and the robot for loading wafers fromthe atmospheric side into the two loadlocks are listed in the left handcolumn. The operation of each of these components at any one time islisted in the shaded boxes immediately to the right of each listedcomponent. The letters included in these boxes refer to the location towhich the component has travelled at any particular time. For example,the box containing (c) in the line indicating the position of the lowerarm means that, at this time, the lower arm is at the e-chuck. Theletter (M) in the line for the robot refers to a magazine on theatmospheric side supplying wafers to the implant apparatus, and theletters (or) in the line for the robot refer to an ion orientationapparatus for correctly orientating the wafer before it is placed in theloadlock mechanism 2.

The operation of the apparatus can most clearly be described byreferring to the passage of a single wafer (hereafter referred to as thewafer in question) through the apparatus from the time that an untreatedwafer leaves the magazine (M) to the time that the treated wafer isreturned to the magazine (M). It should be understood that, every timethe wafer is deposited at a particular location, the wafer which is onestep ahead of the wafer in question will just have been removed fromthis location. Also each time the wafer is picked up from a particularlocation, it will be replaced by a later wafer which is one stage behindin the process.

The wafer in question is picked up by the atmospheric robot from themagazine (M) and transferred to the orientation mechanism (or) where itis rotated to the correct orientation as shown at 40 in FIG. 5. On itsnext pass, the atmospheric robot picks the wafer in question out of theorientation mechanism (or) and transfers it to the lower loadlock 4. Atthis time, the apparatus has the lower slit valve 25 open and the lowerlid valve 26 raised. Once the wafer in question is in place, the slitvalve 25 is closed and the loadlock chamber 27 is evacuated as shown at41 in FIG. 5. It will be understood that the atmospheric robot loads theupper 3 and lower 4 loadlocks alternately as shown on the bottom line ofFIG. 5.

Once the loadlock chamber 27 is evacuated, the lower lid valve 26 islowered by the cam mechanism 24. The wafer in question is now in aposition in which it can be gripped by upper gripper arm 29 as indicatedat 42 in FIG. 5. As mentioned above, it will be understood that thelower arm 22 then moves a treated wafer in the opposite direction intothe lower loadlock 4 as indicated at 43 in FIG. 5. The upper gripper arm29 with the wafer in question then rotates about axis 23 towards thee-chuck and waits. While this is happening, the lower gripper arm 22which is now not carrying a wafer moves to the e-chuck and picks up thewafer which has just been scanned as indicated at 44 and FIG. 5. Thewafer in question is then put onto the e-chuck as indicated at 45 inFIG. 5. The e-chuck is then electrostatically activated to attract thewafer in question to the chuck, and is rotated from its horizontalloading configuration to a vertical scanning configuration which takesapproximately one second and is illustrated at 46 in FIG. 5. The waferin question is then scanned with the ion beam as previously describedand as indicated in 47 in FIG. 5. Once this operation is complete, thee-chuck returns to the horizontal loading configuration as indicated by48 in FIG. 5, whereupon the lower gripper arm 22 retrieves the wafer inquestion as illustrated at 49 in FIG. 5. The upper gripper arm 29 loadsthe next wafer to be treated onto the e-chuck as indicated at 50 in FIG.5. The lower arm then rotates about axis 23 and transports the wafer inquestion to the lower loadlock 4 as indicated at 51 in FIG. 5. At thistime, the lower lid valve 26 is in its lowered position and the slitvalve 25 is closed. Once the wafer in question is in place, the lowerlid valve 26 is raised and the loadlock chamber 27 is vented back toatmospheric pressure as indicated at 52 in FIG. 5 through port 28, or aseparate port. Once the chamber has been vented, the slit valve 25 opensand the wafer is collected by the atmospheric robot and returned to themagazine containing completed wafers.

As is apparent from FIG. 5, while one of the loadlocks 3, 4 is beingpumped to vacuum, the other is being vented to atmosphere almostsimultaneously, but slightly later. This means that while the treatedwafer is being transported out of the vacuum chamber through oneloadlock, an untreated wafer is being transported in through the other.This allows a regular supply of wafers to the e-chuck, thereby reducingthe gap between implant operations.

With the apparatus of this invention it will be possible to process upto 270 wafers per hour, as opposed to about 200 per hour in the priorart.

What is claimed is:
 1. A robot for transferring planar members from onelocation to another, the robot comprising: a pair of coaxial shaftshaving a common axis extending, in use, perpendicular to the plane ofthe planar members, each shaft having, a respective rigid robot arm armattached thereto carrying a respective gripper for gripping a planarmember, at a fixed radial distance from said common axis, the two shaftswith said respective attached robot arms and grippers being rotatableindependently about the axis and movable together along said axis, and asource of motive power for providing the axial and rotational motion. 2.A robot according to claim 1, wherein each gripper has a pneumaticmechanism for opening and closing the gripper to respectively releaseand grip a said planar member, and the coaxial shafts contain air ductsfor the supply of air to actuate said pneumatic mechanisms.
 3. A robotaccording to claim 1, including a lead screw operable to effect saidmovement of the coaxial shafts along said axis.